1. Field of the Invention
The present invention relates to the conversion of gaseous carbon dioxide into an aqueous solution of alkaline and/or alkaline earth bicarbonate solutions useful for the production of solid alkaline bicarbonates, alkaline carbonates, and/or alkaline earth carbonates. In a preferred embodiment, the invention entails converting a feed stream of gaseous greenhouse carbon dioxide into an alkaline and/or alkaline earth bicarbonate solution using solid regenerable inorganic cationic exchanger materials, specifically either crystalline or amorphous silicoaluminates.
2. Description of the Prior Art
Most of the solid alkaline bicarbonates and carbonates are obtained from their aqueous solutions, by any evaporation or crystallization process. These bicarbonates and carbonates have numerous applications in daily life. To be mentioned, two of the most used are sodium carbonate and sodium bicarbonate.
Sodium bicarbonate (NaHCO3) is one of the most widely used chemical commodities in the world. It is used in cooking as a leavening agent; in medicine as an antacid to treat chronic forms of metabolic acidosis and in cardiopulmonary resuscitation; in skin defoliants, toothpastes, cleaning agents, chemical fire extinguishers, and fungicides; for the biological control of wastewater; and as a dyeing agent in the textile industry.
Another alkali material, synthetically derived from aqueous solutions of sodium bicarbonate, is sodium carbonate (Na2CO3) or soda ash, which is used in large amounts in making glass, sodium silicates, soaps and detergents, and for flue gas desulfurization, among other things. Lithium carbonate and potassium carbonate are used in molten carbonate fuel cells. Alkaline earth metal carbonates are widely used in the synthesis of ceramics, as catalytic support for the preparation of carbon nanotubes, in the paper industry, and for cleaning solid surfaces by dry blasting. All of these alkaline bicarbonates and alkaline or alkaline earth carbonates are prepared using their aqueous solution as raw material. In particular, all of these sodium bicarbonate and sodium carbonate chemicals are prepared using an aqueous solution of alkaline or alkaline earth bicarbonate as raw material.
The main naturally occurring source of sodium bicarbonate is nahcolite (NaHCO3), but it occurs frequently in association with trona mineral—trisodium hydrogendicarbonate dihydrate or sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). One way to get a sodium bicarbonate solution is through a carbonation of a dissolution of trona mineral, after several processes for removing the impurities present in trona. However, high-quality reserves of sodium bicarbonate are unevenly distributed around the world and are a mineral resource that is being largely depleted. They are found mainly in Utah, California, Colorado and Wyoming, which makes this resource difficult to obtain in many developed and developing countries. Similarly, potassium bicarbonate solutions are obtained by bubbling carbon dioxide in a solution of potassium carbonate, as described in Ullmann's Encyclopedia of Industrial chemistry, Vol A5, fifth edition. Wolfgang Gerhartz (exe Ed), pp 173-174. New York, 1986.
A sodium bicarbonate solution can be obtained through the Solvay process using carbon dioxide. Also called the ammonia-soda process, this is a major conventional process for producing a synthetic sodium bicarbonate solution, which is the source for making refined sodium carbonate, Na2CO3 (soda ash), refined sodium bicarbonate, NaHCO3, and occasionally refined sodium sesquicarbonate, Na2CO3.NaHCO3.2H2O. In the Solvay process, carbon dioxide (CO2) is dissolved in water containing ammonia (NH3) and sodium chloride (NaCl) to precipitate sodium bicarbonate (NaHCO3), which is then separated by filtration.
Conventionally, this is carried out in two main steps: Ammonia gas is first introduced into sodium chloride brine, and only then, in another apparatus, is carbon dioxide gas introduced to make a sodium bicarbonate saturated solution, from which bicarbonate precipitates. Although the chemical reactions involved may seem simple, they are complex in reality. Considerable heat is evolved during the ammoniation of brine, and intense cooling is required for the necessary degree of ammonia saturation. Carbonation of ammoniated brine is also accompanied by the evolution of considerable heat, so that the apparatus must be cooled in order to improve yield, and the cooling must be controlled to form good crystals. Thus, the process is complex and intensive in energy use.
Because of the ammonia, the obtained sodium bicarbonate solution is contaminated with ammonium compounds such as ammonium carbonate ((NH4)2CO3), ammonium bicarbonate (NH4HCO3) and ammonium carbamate (NH4COONH2). Most of these impurities end up in the final solid sodium bicarbonate and render it unfit for many uses, and additional refining steps are required for adjusting purity, regardless of the purity of the sodium chloride and carbon dioxide used.
Moreover, the Solvay process is not ecologically friendly; the management and safe disposal of chloride-containing waste streams is one of its major problems. And its cost is so high that many people in developing economies cannot afford sodium bicarbonate produced by this process.
In nature's carbon cycle, falling rainwater, with its high surface area, picks up carbon dioxide as it descends through the atmosphere. When fallen rainwater moves down through the topsoil, it may acquire additional quantities of carbon dioxide produced by the biological decomposition of organic matter. All of that carbon dioxide dissolved in water produces a weak carbonic acid, which can react with calcium carbonate deposits to produce calcium bicarbonate, which is more soluble in water than calcium carbonate. This mechanism increases the calcium content in groundwater. The same can be said for magnesium carbonate deposits, which are also soluble in water.
This natural process establishes typical bicarbonate content in rivers within the range of 30 mg/L to 400 mg/L (Water Quality and Its Control, James C. Lamb III. Chap 7, pp 131-135, John Wiley & Sons Inc. 1985). For the removal of calcium and magnesium bicarbonates to soften water, a practice of longstanding employs materials with cationic exchange properties. Such materials are in general called “zeolites,” which in old field parlance refers to a set of materials with different compositions and structures, including bentonitic clay, synthetic gel-type minerals, lignite, soft coal, bitumens, and synthetic resins (“The Chemical Process Industries” R. Norm Shrive, Chap. 4, pp 47-50, 2° Ed. McGraw-Hill Book Company. 1956). The following reaction represents the use of zeolite to soften water containing calcium bicarbonate:Ca(HCO3)2+Na2Z→CaZ+2NaHCO3 A similar reaction may be written for other bicarbonates found in water, such as those of magnesium, calcium, etc.
However, a process in accordance with the above reaction is not efficient for producing large quantities of sodium bicarbonate from river water. Considering the low concentrations of bicarbonate mentioned above, it would require an enormous amount of time to get a solution with an appreciable concentration of sodium bicarbonate. The sodium bicarbonate would moreover be highly contaminated with bicarbonates of other cations, such as magnesium. Also, it must be realized that it is at least a two-step procedure, where the first step is carried out by nature—carbon dioxide captured by rain—and the second is a man made exchange using a sodium zeolite.
In U.S. Pat. No. 2,392,575 dated 1946, Tiger et. al. disclose a metathetical two-step conversion of common salt to sodium bicarbonate (NaHCO3) through the net reaction:NaCl+H2CO3═HCl+NaHCO3 This reaction is performed using an insoluble hydrogen “carbonaceous zeolite” (acid-treated sulfate lignite or soft coal) stripped of metallic cations. First, a sodium chloride solution is passed through a bed of such hydrogen carbonaceous zeolite. An excess of sodium chloride and hydrochloric acid is produced, leaving a sodium carbonaceous zeolite. Second, the sodium is contacted with a pressurized solution of carbonic acid obtained as a product of combustion. The hydrogen carbonaceous zeolite is regenerated, and a dilute solution of sodium bicarbonate is produced.
The hydrogen carbonaceous zeolite, the sodium chloride solution, and the solution of carbonic acid cannot be put all together in one simple step, because the sodium chloride exchange produces hydrochloric acid (HCl), which, together with the hydrogen carbonaceous zeolite, which is also acidic, would quickly decompose the sodium carbonate of the solution of pressurized carbonic acid into carbon dioxide gas (Chemical Characteristics and Structure of Cracking Catalysts, A. G. Oblad, T. H. Milliken, Jr., and G. A. Mills in Advances in Catalysis vol. 3, p 204, Academics Process, 1965). The high acidity level (a pH less than 4) hinders the formation of bicarbonate regardless of the length of time of the contact with the pressurized solution of carbonic acid.
There are several disadvantages with these relatively inexpensive hydrogen carbonaceous zeolites: low cationic exchange capacity (less than 1 meq/g), poor mechanical resistance (not suitable for pressurizing, bubbling, or a fluidized bed), and poor chemical stability to alkalis (sodium bicarbonate or sodium carbonate solutions), so that carbon particles go into the bicarbonate or carbonate solution and then peptize (F. Helfferich. Ion Exchange. Chap 2, pp 17-18. Dover Publications Inc. 1995; M Kodama, N. Shimisu, et al. Carbon 28 (1): 199-205 (1990)). Other types of carbonaceous materials, including cationic exchange resins, have better properties, but their initial cost, currently around $10,000/m3, and regeneration cost both adversely affect the economics.
Amorphous silica aluminas and crystalline zeolites became available around the middle of the twentieth century. Their sodium forms are cheap, have high cationic exchange capacity, and can be used to perform the procedure disclosed in U.S. Pat. No. 2,392,575. To obtain the hydrogen form of such aluminosilicates, first the sodium form must be converted into the ammoniacal form through ion exchange with an ammonium salt solution and dried. Then, a calcination step at a temperature within the range of 400° C. to 500° C. produces the hydrogen form of the aluminosilicate, very often with severe loss of cationic exchange capacity. Only after performing these preparatory steps can the procedure of U.S. Pat. No. 2,392,575 be performed.
In this procedure, a sodium chloride solution is passed through a permeable bed of the hydrogen form of these aluminosilicates. This produces hydrochloric acid, which causes a destructive acid attack on the aluminosilicates and a decomposition of the sodium bicarbonate as mentioned above. So in this procedure, the hydrogen form of these aluminosilicates does not improve the production of a sodium bicarbonate solution.
The prior art processes for obtaining sodium bicarbonate solutions clearly leave much to be desired.